Abstract: Through investigation of tracking infected Spodoptera litura larvae,a study was carried out to observe the epizootic dynamics of nuclear virus disease of S. litura after virus release in different instar with concentration of 2.15?10⁷PIBs穖l^-1 in vegetable field. The results showed that with the age of S.litura increasing,the susceptibility to virus,mortality and death velocity decreased,the disease death sustained period of larvae obviously elongated,and the time that larval mortality and diseased prevalence began and peaked were postponed. The distribution of disease death time could be well simulated by Complementary log log model (CLL). Hosmer Lemoshow test showed that the theoretic values well fitted to observed data,and t test indicated that the coefficients of the model reached significant level. The distribution of diseased prevalence of host larvae was simulated by Holliday model,the regression of function was significant by Ftest,and its coefficients reached or approached significant level. Moreover,the cumulative diseased death time distribution could be described by Stype models (Gompertz,Logistic,Weibull),in which,the Gompertz model was the best.

Key words: Spodoptera litura, Larval instar, Nuclear polydedrosis virus, Disease epizootic, Distribution of disease time, Complementary log log model